Electronic compressor control



Dec. 11, 1945.

w. s. THOMPSON 2,390,856

v ELECTRONIC COMPRESSOR CONTROL Filed March 24, 1942 Sheets-Sheet 1 6 7 6 l 10 JMPLIHER M/xEe AMPLIFIER Canpezssoz Zi g 5 E AMPuF/Ee 13 J2 fie ce/E052 AMPLIFIEE 77445 COM/ 25550 77ME L/M/TEE W21; TEE 5, Yi/OMPSDAQ INVENTOR WKWW ATTORNEY Dec. 11, 1945. w. s. THOMPSON 2,390,856

ELECTRONIC COMPRESSOR CONTROL 7 Filed March 24, 1942 2 Sheets-Sheet 2 WWW I v n nu. li

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In INVENTOR I 1 ATTORNEY actually takes place.

Patented Dec. 11, 1945 ELECTRONIC COMPRESSOR CONTROL Walter S. Thompson, Los Angeles, Calif., assignor. to Radio Corporation of America, a corporation of Delaware Application March 24, 1942, Serial No. 436,005

11 Claims.

This invention relates to electrical transmission systems, and particularly to an electrical compression system for sound recording, radio broadcasting or public address systems.

It is well known that electrical currents are compressed for various reasons, compression being used in the recording of sound for motion pictures, to increase the film volume range, to aid the mixer and for various other reasons well known in the art. Thus, the use of compressors in sound recording channels is practically standard practice, one such compressor system being disclosed and claimed in U. S. Patent 2,255,683 of September 9, 1941 to Kurt Singer.

In the operation of such a compressor unit, the signal must first pass through avariable gain amplifier, one or more linear amplifiers, and a rectifier, before reaching the grids of the variable gain amplifier tubes at which point compression To obtain the necessary filtering between the rectifier and the grids of the variable gain amplifier tubes, it is necessary to use various circuit elements such, as resistances and capacitances, and sometimes inductances, any of which introduce a delay in the compression action. If this filter .delay is too short, there is a tendency for the system to oscillate.

I'hus, compressors of the type above-mentioned have a delayed action, the result of which is to permit the beginning of certain signals to pass through the channel and be recorded or reproduced without compression. This introduces a distortion into the transmitted signal which is quite disagreeable, as it has a tendency to emphasize the first syllables of words.

The present invention is directed to a system for insuring compression action at all times, and

particularly to the first few cycles at the begin-' ning of any sound wave. This is accomplished in general by employing in the sound recording channel an instantaneous limiter which operates immediately upon the first cycle or cycles of any sound wave being transmitted therethrough havflatten the tops of the first cycle or cycles of the waves, this distortion is not deleterious in comparison with that obtained when these cycles are not compressed.

'The principal object of the invention, therefore, is to facilitate and improve the compression of electrical currents.

Another object of the invention is to insure compression action over the entire portion of a signal at which compression is desired.

A further object of the invention is to provide a compression transmission channel having an instantaneous limiter thereon for introducing compression during the inoperative period of the compressor.

A further object of the invention is to provide a combination compressor and instantaneous limiter in a transmission circuit, the instantaneous limiter being under control of the compressor, during the functioning of the compressor.

Although the novel features which are believed to be characteristic of this invention are pointed out with particularity in the claims appended herewith, the manner of its organization and the mode of its operation will be better understood by referring to the following description read in conjunction with the accompanying drawings, forming a part hereof, in which:

Fig. 1 is a block diagram of a sound recording channel embodying the invention;

Fig. 2 is a schematic circuit diagram of the compressor and instantaneous limiter used in the system of Fig. 1;

Fig. 3 is a graph showing the operation of one of the instantaneous limiter tubes:

Fig. 4 is a graph of a sine wave, illustrating the action of the compressor and instantaneous limiter thereon; and, 4

Figs. 5 and 6 are graphs showing the timing of the compressor and instantaneous limiter.

Referring now to Fig. 1, a microphone 5, or

other source of signal currents, feeds a preamplifier 6, which is connected to a mixer I feeding a second amplifier 8. The amplifier 8 is connected to a compressor 9 of the type referred to in .the above mentioned Singer patent, the output of ing a monitoring speaker IS. The above system is a standard type of sound recording channel with the exception of the limiter HI, although this type'of transmission channel may also be used in 7 radio broadcasting or public address systems.

Referring now to Fig. 2, it will be observed that the compressor unit is of the type shown in the above-mentioned Singer Patent 2,255,683 and will,therefore, not be described in detail. In operation, a signal is impressed over audio transformer to variable gain tubes 2| and 22, the output of which is fed to a linear push-pull amplifier having tubes and 25. A portion of the output of the tubes and 25 is fed over conductors 28 and 29 to an amplifier 26 and then to a rectifier 21, the rectified current controlling the gain of tubes 2| and 22, as described in detail in the Singer patent. In the operation of the compressor, the breakaway point or point at which compression begins is determined by the potentiometer 30 while the date at which compression occurs is determined by the potentiometer 3|.

As mentioned above, it requires a certain amount of time for the compression actionto become eil'ective, and this is true .even when the amplifier 26 is connected to the input of the vari able gain amplifier 2I22. To illustrate what occurs to a signal in such a compressor, reference is made to Fig. 4, wherein a sine wave of difierent amplitudes is shown. In this illustration, the breakaway or compression point has been set at a level corresponding to the peaks of the first cycle a. Thus, with the normalcompressor, the first cycle b-c, of greater amplitude than cycle a, will pass to the recorder without compression, while the succeeding cycle d will receive compression as shown by the full line curve. Since the time period in which it takes the compressor to become fully efiective is substantially constant, the number of peaks of any particular wave which will pass through uncompressed will depend, of course, on the frequency of the wave. As will be shown hereinafter, the present invention compresses all peaks above the predetermined compression point.

Referring again to Fig. 2, it will be noted that the output transformer 33 feeds a pair of pushpull amplifier tubes as and at having: resistors tube, since the two tubes are functioning in a push-pull relationship. Thus, the complete cycle b'c' will be flattened and compressed. Although it is realized that the flattening of the peaks of cycle b-- c' does 'not exactly correspond to the- The effect of this flattening of the peaks is' shown in Fig. 4, by the dotted lines within the cycle bc. This type of compression is not, of course, desirable for the entire signal, and it is,

therefore, necessary that the normal type of compressor action be substituted to retain the original wave form of the wave. However, by the stand 31 in their grid circuits. In the common grid conductor for these tubes there is a fixed bias potential provided by a battery 3Q or other suitable source in series with a resistor at? The point M on the common grid return for the tubes 38 and 35 is connected by a conductor 82 to the rectifier 27 at point 63. By this connection, the instantaneous gain of the tubes 35 and 35 is controlled simultaneously with the control of the normal gain of tubes 2i and 22 by the output of the rectitiometer 30, the operating point for the tubes 3% and 35 is placed at point B. Thus, the first cycle a of the signal of Fig. 4 will be transmitted through the tubes at and 35 without compression or distortion as shown by the sine waves a and a" in Fig. 3. However, when the cycle 12-0 of Fig. 4 is impressed on the tubes 3d and 35. the result is shown at b"- -c", which result is the flattening of the half-cycle b as shown at b". Although only the half-cycle b is shown flattened in Fig. 3. it is realized of course, that the half-cycle c' will receive the same flattening action in the other represent the time that'cycle a is being trans-- mitted, while the sloped portions 1 and f? of the curves in a horizontal direction, represent the time cycle b-c is being transmitted. Thus, with the time period made equal to the period f, the limiter action is eliminated during the same time interval that is required for the compressor to become effective. Furthermore, by selecting the proper values of grid bias 39 and resistors ti: and 3? with respect to the operating characteristics of tubes 2i and 22, the same rate of change may be obtained in the compressor and limiter,

. but in an opposite senseas shown by the slopes of portions f and f. Thus the amount of flattening is made comparable to the amount of compression (see Fig. 4)

It will be noted that tubes 2i and 22 and tubes 36 and 85 linearly amplify all signals having amplitudes below the compression point, but when greater amplitudes are transmitted, the effective gain of tubes 34 and 35 instantaneously drops to a point corresponding to the eventual gain of the compressor. However, the rectifier output caused by cycle b-c immediately begins to vary the gain of the compressor tubes and simultaneously shift the instantaneous gain of the limiter tubes at the same rate but in an opposite sense, so that when cycle d reaches the compressor, compressing action is fully efiective and tubes 3 and 35, operating about point C, linearly amplify or transmit without amplification, all signal levels. I

It will be observed from the above combination of compressor and instantaneous limiter. that the compressor can now be provided with all filtering necessary between the rectifier and grids of the variable gain amplifier tubes, since the instantaneous limiter will provide any time interval desired. Although the limiter has been shown controlled by the compressor, it is to be understood that it may be self-controlled or controlled by a rectifier separate from the compressor rectifler so that the limiter may be rendered ineffective over a time interval different from that of the compressor.

what I claim as my invention is:

1. In an electrical transmission system, the

combination of means for generating electrical currents, means for compressing the ampltiudes of said currents in a predetermined amount, said compressing means having a delayed action which permits the beginning of said currents to be transmitted therethrough without compression, a second means for compressing the beginning of said currents during the inoperative period of said compressing means, and means for simultaneously controlling both of said compressing means, said last mentioned means mak-= ing said first compressing means operative while making said second compressing means inoperative over the same time periods. 1

2. An electrical transmission system in accordance with claim 1, in which said first men a biasing current for said compressor unit, and a current limiter connected to the output circuit of said compressor unit for transmitting said signals and to the output circuit of said rectifier, for transmitting said rectifier currents to said limiter for biasing said limiter to a non-limiting condition while said compressor is being biased to a compressing condition.

5. The method of compressing a predetermined signal range of amplitudes with a compressor circuit requiring a certain time period for the signal to introduce a predetermined amount of compression into said signal, comprising instantaneously limiting the transmitted amplitudes of said range externallyof said compressor circuit and varying the effectiveness of said limiting action during said certain period required for said compression action to become effective, said limiting action being totally ineifective when said signal has introduced said predetermined amount of compression into said signal. I

6. The method in accordance with claim 5 in which the limiting action is decreased at the same rate and over the same time period required for said compressing action to become eil'ective.

7. In combination, a signal transmission line, a delayed action amplitude limiting device in said line for limiting the amplitude of transmitted signals according to a predetermined ratio, a secondinstantaneous amplitude limiting device for limiting the amplitude of the portion of said signals not limited by said first device, and means acting on said devices for simultaneously eliminating the limiting ability of said second device while making eilective the limiting ability of sad first device.

8. In combination, a signal transmission line, a compressor including a variable gain amplifier, a rectifier, means for connecting the output of said variable gain amplifier to the input oi said rectifier, means for connecting the output of said rectifier to the gain control electrodes of said variable gain amplifier, the above mentioned elements forming a compressor unit, a limiting amplifier connected to the output of said variable gainamplifier, and means for connecting the output or said rectifier to the input of said limiting amplifier, the output of said rectifier introducing compression action in said variable gainamplifier as said limiting amplifier is made nonlirniting.

9. in, combination, an incoming transmission line, an outgoing transmisison line, a variable gain amplifier having gain control electrodes and I a limiting amplifier having gain control electrodes, said amplifier being connected in series producing a current substantially roportional to the average amplitude of said signals, means for connecting the output of said rectifier with the gain control elements of said variable gain amplifier, a second limiting amplifier for said signals connected to the output of said variable gain amplifier, and means for connecting the output of said rectifier to the control electrodes of both of said amplifiers, said second amplifier reducing the amplitude of said signals from a maximum to a minimum amount as said variable gain amplifier reduces the amplitude of said signals from a minimum to a maximum amount over a predetermined initial time period.

11. An electrical current transmission system comprising a source of electrical currents, a delayed action compressor unit for said currents, an instantaneously acting limiting device for said current, and means for interconnecting said unit and device for compressing said currents with substantially no distortion, said limiting device being made inoperative .as said compressor unit becomes operative.

WALTER S. THOMPSON. 

